Magneto-inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement: charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes likewise arranged essentially perpendicularly to the flow direction of the medium. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube; it is, thus, proportional to volume flow. The measurement voltage is usually tapped via a measuring electrode pair, which is arranged in the region of maximum magnetic field strength, where, thus, the maximum measurement voltage is to be expected. The measuring electrodes are coupled with the medium either galvanically or capacitively.
In the case of application of medium-contacting, measuring electrodes, there form, at the interface between the metal measuring electrode and the medium flowing through the measuring tube, galvanic elements, which cause an electrochemical disturbance potential. This electrochemical disturbance potential varies over time, since it depends on different, changing, environmental conditions, such as temperature, pressure, composition of the medium, material of the measuring electrodes and material of the measuring tube. Thus, for example, the composition of the surface of the measuring electrodes can change as a result of formation of a passivating layer or as a result of corrosion. The varying electrochemical disturbance potential superimposes itself on the actual measurement voltage proportional to the flow velocity of the medium flowing through the measuring tube. It is clear, that an electrochemical potential changing as a function of time influences negatively the accuracy of measurement of a conventional, magnetically inductive, flow measuring device. Methods are, therefore, needed, which eliminate these disturbance signals. It is especially critical, when the medium to be measured is a medium having a small electrical conductivity flowing through the measuring tube with a relatively high flow velocity. Due to the influence of the relatively large disturbance voltage on the measurement voltage, there is then the danger, that the measurement voltage will disappear in the noise, whereby a reliable and repeatable, flow measurement becomes impossible.